Membranes for the separation of gas mixtures by selective permeability have been in commercial use for a considerable period of time. A common form of such a membrane is a hollow hair-like fiber, with a bundle of such fibers, numbering as many as several hundred thousand to well over a million, joined together into a single unit where the fibers are arranged in parallel. The units are shaped as cylindrical modules, with the fibers arranged parallel to the cylinder axis and both ends of the fibers embedded in tubesheets formed from potting compounds. The tubesheets fix the positions of the fibers relative to each other and to the module casing, and direct the flow of incoming gas to the fiber lumens in modules designed for boreside feed, or to the fiber exteriors in modules designed for shellside feed, with the permeate being drawn from the shellside or boreside, respectively.
The formation of the fibers is a critical process, since the effectiveness of each fiber as a separation medium is determined not only by the polymer used but also by the liquid medium in which the polymer is dissolved for extrusion and the manner in which the liquid is removed subsequent to extrusion. Conventional manufacturing methods involve a liquid medium consisting of a mixture of two nonaqueous liquids, one of which is a solvent for the polymer and the other a non-solvent. Descriptions of such mixtures and their use are found in numerous patents, examples of which are Jensvold, J. A., et al., U.S. Pat. No. 5,141,530 (Aug. 25, 1992), and Sanders, Jr., E. G., et at., U.S. Pat. No. 4,955,993 (Sep. 11, 1990). Once the polymer has been extruded into fibers, a portion of each component of the liquid medium is removed by aqueous quench and leaching baths, which also serve to establish in a preliminary manner the pore structure of the fiber and hence its permeation qualities. The fibers are then wound onto spools or Leesona packages on which the final processing takes place before the fibers are ready for arrangement in bundles and placement in the module casing. This processing involves a prolonged aqueous extraction of the remaining solvent, followed by drying. The fully formed and dried fibers are then placed over a core tube where the fibers are layered to a depth sufficient to include the desired number of fibers and to fill the module casing. The tubesheets are applied either while the fibers are being placed over the core or afterward, and the collected fibers are then encased in the module housing. Descriptions of the application of the fibers to the core, the formation of the tubesheet and construction of the module in general are found in many patents, examples of which are Clark, G. B., U.S. Pat. No. 4,080,296 (Mar. 21, 1978), and Thibos, P. A., U.S. Pat. No. 4,824,566 (Apr. 25, 1989). Each of the patents listed in this paragraph are incorporated herein by reference.
The processing of fibers on spools presents certain difficulties. For example, the degree of exposure of any segment of the fiber to the surrounding environment will vary with the location of that segment in the spool. With many layers of fiber on a single spool, this can produce a lack of uniformity between the innermost and outermost fibers of the spool. To minimize or eliminate this risk, processing continues for an extended period of time, generally 24 hours or more. Furthermore, labor is required to handle the spools and to monitor the processing conditions, space is required to store spools waiting to be processed and spools already processed and waiting to be placed in modules, and a processing chamber is needed of sufficient size to accommodate a large number of spools. A further difficulty is that solvent removal and drying cause the fiber to shrink, and shrinkage on the spool results in pressure being exerted on the fiber segments occupying the innermost part of the spool winding. This crushes the segments, which creates a further source of nonuniformity, and a lowering of the effectiveness of the fiber bundle as a whole.
Placement of the fibers around the core tube is achieved by machinery which folds the fibers into lengths by wrapping them around monofilaments whose spacing corresponds approximately to the length of the fibers in the final module. The lengths are then placed around core tube. This machinery unfortunately consumes an excess of fiber, resulting in wastage of up to about 25% of the fiber. In addition, the fibers have a tendency to slip across the core during the procedure, which detracts from the uniformity of the packing density of the fibers in the finished module.
These and other difficulties are addressed by the present invention.